But, how long do the good things last?

what happens when an old mission that was forgotten in the mess of the war is uncovered. what if they had a chance to go back in time? Hermione, Ginny, Luna, and Lavender are the girls of the prophecy and ready to change the past. but how much can they change before they are doing more hurt than good? And also, what about the other things in the past that come their way.